This invention is concerned with a Rankine cycle system employing the use of cold seasonal temperatures to form an ice bed and insulating said ice bed from warm seasonal temperatures to provide a year round cold source for the system.
A Rankine cycle system comprises an assembly of apparatus which makes use of a difference in temperature of a working fluid to produce useful work. Generally such a system comprises a heat source which heats a primary fluid such as water stored in a boiler, an evaporator containing working fluid having a vapor pressure significantly above that of water in contact with the heated primary fluid, a turbine or other engine for converting the kinetic and heat energy of a gas to mechanical work which is operated by vaporized working fluid under pressure from the evaporator, and a condenser associated with a cold source which condenses vaporized working fluid exhausted from the turbine or engine which condensed fluid is returned to the evaporator to complete the cycle.
The efficiency with which the system produces work is largely a matter of how great a difference in temperature exists between the hot and cold sources. The theoretical Carnot efficiency is governed by the equation: ##EQU1## wherein H.S. (.degree.K) equals the temperature of the hot source in degrees Kelvin and C.S. (.degree.K) equals the temperature of the cold source in degrees Kelvin. The greater the difference in temperature between the hot and cold sources, the higher the efficiency, viz the larger the percentage of heat energy is converted into work.
In the past, this temperature difference was made as large as possible by burning a fuel such as oil or coal for the heat source of the system and keeping the other side as cool as possible, usually with running water. While the effects of burning a fuel are obvious for providing high temperatures to the heat source the temperature of the cold source is also important and has heretofore been largely overlooked.
Historically, there has never been any serious problem meeting both the heating and cooling requirements for these systems. However, this may not be true in the near future. With the supply of petroleum in question and its eventual exhaustion no longer doubted, a serious problem is presented. Alternate sources of heat such as nuclear, coal, or even wood offer no immediate solution. More exotic alternatives such as geothermal, ocean thermal, solar, wind etc. are either wholly inadequate or severely limited in the extent to which they can be applied. In all cases except nuclear, the prohibitive factors are economic and/or severe geographical restrictions.
With fuel supplies dwindling, it would seem inevitable that Rankine cycles, as well as all heat engines, may receive less and less attention as energy suppliers. However, there yet remains a number of natural environmental sources having temperature differences which can provide near infinite amounts of energy and can be used for Rankine cycle systems. The main drawback to their use lies in the small temperature differences which they offer.
One such system is the government-sponsored project OTEC, or Ocean Thermal Exchange Cycle, which makes use of small differences in temperature found at different depths in some parts of the ocean. Such systems are described in U.S. Pat. Nos. 4,104,883 and 3,986,622.
Another system utilizes the inherent temperature differences of areas of the earth's surface where there are relatively large elevational differences, e.g. a mountain top and the valley below it. Such systems are described in U.S. Pat. No. 3,953,971.
While there exists some debate over what efficiencies can actually be achieved by these systems, even the most optimistic estimates place it at only a few percent due to the relatively small temperature differences.
Moreover, when a small difference in temperature must be used, the walls of the evaporator of the system do not transfer heat rapidly and this quickly causes enormous problems both technically and economically. Because the rate of heat transfer is slow, the surfaces of both the evaporator and the condenser must be increased hundreds or even thousands of times. So, what was originally a simple evaporator and condenser connected through a turbine, becomes a series of enormous pressurized heat exchangers and the cost of the system vastly increases. Add to these problems, severe geographical restrictions and the survival of these systems in rough weather, and it becomes apparent, what difficulties these systems present.
The temperature of the condenser in contact with the cold source is as important as the temperature of the evaporator in contributing to the efficiency of a Rankine cycle system. If the condenser is cooled significantly, e.g. to 0.degree. C., while maintaining the evaporator at room temperature, the turbine or engine can still perform by drawing heat away from the environment. If the condensed working fluid is recycled back to the evaporator, the system will continue to do work as long as the condenser is cooled. If the evaporator is maintained at about 100.degree. C. and the condenser at 0.degree. C., the Carnot efficiency is about 27%. A steam engine, on the other hand, having a boiling temperature of 200.degree. C. and a condenser temperature of 100.degree. C. would have a Carnot efficiency of about 21%. The efficiency of a nuclear power plant is only about 25%.
Thus, it can be seen that by providing a moderately high hot source for a Rankine cycle system and a cold source at the temperature of ice, 0.degree. C., a reasonably efficient system can be obtained.
The present invention provides a Rankine cycle system, employing ice as the cold source, which ice is generated using ambient temperatures available during the cold seasons and stored for use during warmer weather. The hot source can be solar heated water or waste heat from commercial facilities which achieves temperatures of 80.degree. C. or above.
In other aspects of this invention improved apparatus for use in a Rankine cycle system are provided including an evaporator, turbine assembly, and condenser.
To further increase efficiency a freezing point depressant such as an inorganic salt may be added to the ice preferably after it is formed to lower its temperature as low as -60.degree. C., if the cold season supports such temperatures.